Control for pulverizing mill



April 22, 1958 H. c. MITTENDORF ET AL 2,831,637

CONTROL FOR PULVERIZING MILL Filed Dec. 1, 1953 2 Sheets-Sheet 1INVENTORS Harvey C. Mittendorf Konrad S. Svendsen ATTORNEY -H. c.MITTENDORF ET AL 2,831,637

April 22, 1958 CONTROL FOR .PULVERIZING MILL 2 Sh eets-Sheet 2 FiledDec. 1, 1953 4 INVENTORS Harvey C. Mittendorf Konrad S. Svendsen JT'I'OR NE Y CONTRQL FOR PULVERIZlNG NULL Harvey C. Mittendorf, EastGrange, N. J., and Konrad S. Svendsen, Eellerose, N. V assignors toCombustion V Engineering, lino, New Yorlr, N. Y;, a corporation ofDelaware This invention relates to control systems which regulate theamounts of fuel and air delivered for combustion to the furnaces ofvapor generating units; and is specifically concerned with controllingthe amount and temperature of primary air conveying the fuel from apulverizing mill to the burners into the furnace of a vapor generator.

The invention can be applied with great benefit to a steam generatingunit having a furnace subjected to variations in static pressure withfluctuations in steaming load.

If a furnace of this type is fired with pulverized fuel for varyingsteam generating loads, the amount of primary air, i. e. air conveyingthe pulverized fuel entrained therein from the mill to the burners intothe furnace, must bear a preferred quantitative relationship with theamount of coal delivered to the furnace. This relationship directlyar'fects the grinding capacity of the mill, the conveying capacity ofthe fuel pipes from mill to burners and the performance of the burners.

Accordingly the proper relationship between amount of coal and amount ofprimary air depends primarily on a coal-air mixture velocity maintainedin the pulveriz ing mill, coal piping and burner that permits theseunits to deliver optimum performance at all loads. For a given flow areaand coal loading this velocity is determined by the volume of primaryair, which in turn depends on the temperature of the air-coal mixtureleaving the mill.

l-leretofore the amount of primary air for each pound of coal burned hadbeen controlled in part by measuring the static pressure at the milloutlet. However such a control system does not take into proper accountfluctuations in pressure at the mill inlet as may occur in millsoperating under pressure, and/or fluctuations in pressure in the furnacechamber as may occur in pressurized furnaces.

It is accordingly 2. primary object of the invention to provide animproved control system for regulating the amount and temperature of theair conveying the pulverized fuel from the mill to the burners into thefurnace under widely varying operating and load conditions.

It is a further object of the invention to provide improved controlmeans for regulating the air flow from mill to burners which controlmeans are responsive in part to variations in steam generating load andresponsive in part to variations in pressure differential between thestatic pressure in the mill and that in the furnace.

It is an additional object of the invention to provide improved controlmeans for regulating the air fiow from mill to burners which controlmeans are also responsive to the quality of the fuel being pulverized asreflected in the heating value and moisture content thereof.

Other objects and advantages will become apparent from the followingdescription of illustrative embodiments of the invention when read inconjunction with the accompanying drawings wherein:

Fig. 1 is a diagrammatic representation of apparatus embodying theinvention applied to a mill supplying fuel to the furnace of a steamgenerator.

Fig. 2 is a portion of Fig. 1 modified to show another form of theinvention.

In carrying out the invention in one form thereof, there is providedcontrol means responsive to the steam pressure prevailing in the steamoutlet header which pressure is indicative of fluctuations in steamgenerating load. These pressure responsive control means serve a dualpurpose: first, to regulate the flow of fuel to the pulverizing mill,and second, to maintain a predetermined ratio of primary air to fuel inthe mill and in the fuel pipe by controlling the amount of primary airentering the mill. The invention further provides corrective controlmeasures imposed upon the aforesaid primary air control means. Thesemeasures comprise apparatus which is responsive to the pressuredifferential existing between the static pressure in the mill and thatin the furnace,

said pressure diiferential being an indication of the volumetricquantity of the air-fuel mixture flowing from the mill through the fuelpipe and burner into the furnace for combustion. The invention furtherprovides for control means responsive. to the temperature of saidair-fuel mixture as it leaves the mill for the purpose of regulating thequantity of hot air required for fuel drying, such quantity being aportion of the primary air entering the mill in controlled amounts asabove outlined.

Referring now to Fig. 1 the apparatus includes a steam generator 1 andits associated furnace 2 both shownin diagrammatic fragments, amill 3, amill fan 4, an 'air preheater 5 and a main air fan 6. Fanj6 deliverscold air via conduit 7 into the air heater 5, the heated air leaving viaconduit 8 to be delivered into the furnace 2 adjacent the fuel nozzle 9of the burner 10. A duct 11 connects into the cold air conduit 7 and aduct 12 connects into the heated air conduit 8, the two ducts joining atright angles into a common duct 13 which is the suction line ofmillfan-4. p

A damper 14 is mounted -at the junction of ducts 11 and 12 in such amanner that its movement will vary the opening to either duct toproportion the relative amounts of cold and heated air flowing into thefan duct 13 and thereby regulate the temperature of the mixture. Adamper 15 in fan duct 13 varies the resistance to airflow through theduct and thereby the amount of primary air. Fan 4 delivers the mixtureinto the mill '3 via duct 16. Obviously the amount of primary airflowing through duct 16 can also be controlled by other means such asregulating the speed of fan 4 by employing a variable speed motor (notshown). After the heated air mixture has passed through the mill'3wherein it serves as-a drying and a transporting medium for the fuelbeing pulverized, the fuel-air mixture leaves the mill 3 via duct 17 andis delivered into the furnace 2 through burner fuel nozzle 9. j I

The mill3 is'provided with a fuel feeder 18 which d elivers the raw fuelin measured quantities into the mill 3. The feeder 18 may by way ofexample be operated in a well known fashion by a motor drivenoscillating pawl which engages one or more teeth of a ratchet wheel (notcrease or decrease the rate of fuel fed to the mill 3.

The apparatus here illustratively shown for controlling the fuel supplyto the mill 3 includes a Bourdon tube 21 connected via pipe 22 to thesteam header 23 of steam generator 1, which tube 21 is responsive tothesteam pressure in said header. The moving end 24 of the Bourdontube'21 raises or lowers the stem 25 of a pilot valve 26. The pilotvalve 26 has an air supply inlet 27 receiving air under pressure, an airoutlet 28 and an air connection 29. The valves 30, by moving up or down,regulate the relative sizes of openings leading. into the outlet 28 andinto the connection 29, opening one while closing the other therebycontrolling the air pressure to the connection 29. As the steam pressurein the header 23 increases or decreases from the normal (depending onthe load demand) the air pressure in connection 29 respectivelydecreases or increases.

Pipe 31 conveys the air pressure from the connection 29 of the pilotvalve 26 into a chamber A of a relay 32. The relay 32 has four chambersA, B, C, D. Chamber A is separated from chamber B by a flexiblediaphragm 33 and chamber .C from chamber D by a flexible diaphragm 34.The diaphragms are fastened to and move a rod 35 which rocks a lever 36which is fulcrumed at 37, and acts to open the spring closed pilotvalves 38 and 39 by means of pins 68 and 69 respectively.

Pilot valve 38 when lowered or raised respectively increases ordecreases the flow of supply air under pressure into chamber D and pilotvalve 39 when lowered or raised respectively increases or decreases theflow of air out of chamber D to the atmosphere. The pilot valves 38 and39are arranged to be both closed by spring action for the mid positionof the lever 36. The chamber B is open to the atmosphere through port4%. The chamber C is connected to chamber D via a needle valvedconnection 41 through which the air under pressure in chamber D may leakinto chamber C to equalize the pressures within the chambers after aperiod of time depending upon the extent of the valve leak. A spring 42opposes the downward movement of the diaphragms 33, 34 or rod 35. a

In operation, the relay 32 receives a pressure such as 15 pounds inchamber A when the steam pressure in the header 23 is normal and thespring 42 balances the 15 pounds pressure in chamber A. Chamber Bis atatmospheric pressure, and pilot valves 38 and 39 are then both in closedposition. The pressure prevailing in chambers D and C is accordinglysustained. When the pressure in steam header 23 rises or falls from thenormal due to decrease or increase in the steam generators load, thepressure conveyed via pilot valve 26 and pipe 31 to chamher Arespectively decreases or increases from the 15 pound pressure and thepilot valve 38 respectively remains closed or opens while the pilotvalve 39. respectively opens or remains closed. Consequently with anincrease in pressure in chamber A from 15 pounds, air under pressurefiows into chamber D through valve 38 (to increase the pressure therein)and with a decrease in pressure chamber A, air flows out of chamber Dthrough valve 39 (to decrease the pressure therein).

Chamber D of relay 32 is connected via pipe 43 to apparatus such asbellows 44, which actuates a lever 20 attached to mechanism 18 forfeeding fuel to the mill 3. An increase or decrease in pressure inchamber D causes bellows- 44 to move lever 20 to increase or decreasethe rate of fuel fedto the mill 3. Should for example, the fuel feed beinadequate to maintain the load on the steam generator, the steampressure will fall, the Bourdon tube 21 will cause pilot valve 26 toraise the pressure in chamber A of relay 32 above 15 pounds which willopen valve 38 and in turn will raise the pressure in chamber D and inbellows 44 to cause an increase in fuel feed. Upon return of the steampressure to normal the Bourdon tube 21 will adjust the air pressure inchamber A back to 15 pounds thereby causing the pilot valves 38 actdirectly on bellows 44 causing the rate of fuel feed to increase ordecrease with steam demand over a small load range.

The air pressure at the mill outlet as measured at connection 45 varieswith the load on the steam generator. In conducting the fuel and airmixture from mill 3 to the furnace 2 the mill air pressure must overcomethe resistance to flow through the duct 17 and through burner 9 plus thepressure in the furnace. For general purposes the duct losses and theburner losses can be assumed to vary substantially as the square of theair velocity through the duct. The difference in pressure between themill and furnace which is the equivalent of the duct losses and burnerlosses is measured by a manometer 46 for the various loads of the steamgenerator.

The manometer 46 is constructed to convert the square functionalrelationship between the velocity and flow resistance of the air-fuelmixture from mill 3 through duct 17 into a linear measurement throughoutall loads. One side 47 of the manometer 46 is connected to the mill at45 via a pipe 48 to measure the mill outlet pressure and the other side49 is connected to the furnace 2via pipe 50 to measure the furnacepressure. The resulting movement'of the lever 51 of the manometer 46actuates a pilot valve 52 which is constructed and operates in the samemanner as pilot valve 26 described above and has the same referencenumbers applied to its individual parts. Pilot valve 52 provides a riseand fall of air pressure in the outlet thereof and in pipe 53 connectedthereto which is consistent with a rise and fall in the volume of theair-fuel mixture flowing from the mill 3 through duct 17 into furnace 2.

Air pressure pipe 53 communicates with relay 54 which is constructed inthe same manner as relay 32 described above and has similar referencecharacters. It has its chamber B connected to pipe 53 and therebyreceives the rise and fall of the pressure from pilot valve 52 asdetermined by manometer 46 .and represents variations in the volume ofthe air-fuel mixture flowing from the mill 3 into the furnace 2. Thechamber A of relay 54 is connected via pipe 55 into pipe 43 which inturn communicates with the outlet 41 of relay 32 thereby receiving therise and fall impulses of the air pressure from relay 32 which determinethe rate of fuel feed. The pressures in chambers A and B of relay 54 arearranged to be counter balanced when the proper ratio exists between thefuel fed to the mill and the air in the fuel-air mixture flowing fromthe mill to the furnace as is reflected in the air pressure in pipe 53.

When such proper ratio exists the lever 36 in relay 54 is in midpositionat any load. Both pilot valves 38' and 39 are then in closed positionand the pressure in chambers D and C of this relay 54 is sustained. Thispressure is conveyed through pipe 56 from chamber D into a bellows 57which acts to set the damper 15 in the duct 13 to a certain position.This position is such as to cause a proper amount of air-fuel mixture toflow from the mill to the furnace, the manometer 46 delivers an airpressure to chamber B of relay 54 which is equal tothe pressure inchamber A.

Should for example, the air flow from the mill 3 be inadequate for theexisting rate of fuel being fed into the mill, the manometer 46 Willthen cause the pilot valve 52 to deliver a too low relative pressureinto the chamber B of relay 54. The higher pressure in chamber A ofrelay 54 will then tilt lever 36' counterclockwise, open pilot valve 38allowing air under pressure to enter which will raise the pressure inchambers D and C of this relay 54 and in bellows 57 thereby turningdamper 15 to a more open position. The resulting increased air flowthrough the mill 3 will determine a new setting of manometer 46, therebyrelatively raising the pressure in chamber B of relay 54; and if theproper setting of damper 15 is effected in this manner the pressure inchamber B' of relay 54 will equal the pressure in chamber A and pilotvalves 38.- and .39 will be in'closed position-to sustain the pres surein chambers D and C and bellows 57 to thereby maintain the damper 15 inits proper setting.

As stated earlier herein above other means,,not shown,

may be employed to control the air flow through duct 13, fan 4 and duct16 with equally beneficial results. Thus a variable-speed motor (notshown) may be used to drive fan 4, the speed of the motor beingdetermined by a resistor (not shown) actuated by means responsive to airpressure fluctuation in pipe 56.

A thermometric element 58 projects into the mill 3 adjacent its outletand measures the temperature of the air leaving the mill. it actuatesthe piston of a pilot valve 59 which is constructed and operates in asimilar manner as pilot valve 26 described above and has similarreference characters. The valves 30' of this pilot valve 59 move in sucha manner that an increase or decrease of air temperature in the milloutlet causes a corresponding decrease or increase in air pressure inpipe 60 leading from the outlet 29 of pilot valve 59 into the chamberA"of a'relay 61.

Relay 61 is identical with relay 32 described above, it has similarreference characters, and its operation is the same. The pressure inchamber A is maintained at a given value, such as 15 pounds, when thetemperature in the mill is normal. The tension of spring 42" is such,with chamber B" open to the atmosphere, that the diaphragm 33" is heldin midposition, and pilot valves 38" and 39" in closed position, therebysustaining a certain pressure in chambers D" and C". Said chamber D" isconnected by pipe 62 to a bellows 63 which, depending upon the pressure,holds the damper 14 in a certain position.

The damper 14 controls the relative amounts of hot and cold air of whichthe air mixture delivered into the mill 3 is composed, and when thismixture, after drying the fuel within the mill, attains the propertemperature as measured by the thermometric element 58, the damper 14isin the desired position. Should the air mixture contain insufficientheat to adequately dry the fuel the temperature leaving the mill will below and the element 58 will cause the pilot valve 59 to increase thepressure in chamber A" of relay 61. This in turn will increase thepressure in chambers D" and C" and in bellows 63 and the damper 14 willthereby be moved to relatively increase the hot air flow. if the properdamper setting is effected, the temperature of the air leaving the millwill become normal and the pressure in chamber A" of relay 61 willreturn to 15 pounds.

Fig. 2 is a portion of Fig. 1 showing a modification of the invention.As described hereinabove and as illustrated in Fig. 1, the thermometricelement 58 in mill 3 effects an air flow under pressure from pilot valve59, through pipe 60 into chamber A" of relay 61. In turn relay 61effects an air flow under pressure from its chamber' 'D through pipe 62to bellows 63 to move damper 14 for adjusting both the hot and cold airflow. In Fig. 2 damper 14 is replaced by damper 14a to only regulate theamount ofhot air flowing through conduit 12 and damper 65 for thepurpose of separately controlling the cold air which in the Fig. 2organization is taken in from the room through conduit 64 joining bothconduits 12 and '13 at the junction thereof. The check damper 65,pivoted at 66 and counter weighted at 67' permits cold air to flowthrough conduit 64 into conduit 13 to form a mixture with the hot airflowing in from conduit 12.

If the proper setting of damper 14a is effected, the temperature of theair leaving the mill 3 will be normal and the pressure in the chamber A"of relay 61 will be of predetermined value such as 15 pounds for thereasons given earlier. Shoud the air mixture contain insuflicient heatto adequately dry the fuel in the mill 3, the temperature leaving themill will be low and the element 58 will cause the damper 14a to move torelatively increase the amount of hot air in the mixture for thereasonsalsot;

given earlier herein.

As stated earlier .in the description of Fig. ratus, damper 15 inconduit 7 amount of hot and cold air delivered by fan dinto the mill 3.

The arrangement of Fig. 2 permits the mill 3 to operate under a pressurerequired to only overcomethe mill and piping resistance because thepressure at the entrance of conduit 13 is near atmospheric, while in thearrangement 1 appaof Fig. l the mill 3 operates under an additionalpres-.-

amount and temperature of the air flowing through the mill in a novelmanner to satisfy the primary air require-- ments regardless of theactual furnace pressure against which the air is delivered andregardless of the temperature of the air necessary to dry the fuel.

While we have here shown certain preferred embodiments of our inventionas applied to a system including one mill and one burner, our newcontrol facilities o bviously may be modified to apply to a multiplicityof mills and/ or burners serving the furnace of a steam generator or thelike; and wherefor it ,will be understood that changes in apparatus andin combination and arrange ment of parts may be made without departingfrom the spirit and scope of the invention as claimed.

What we claim is:

l. A system comprising a pulverizing mill, a boilerfurnace subjected tovariations in static pressure with av fluctuating steam load, a burnerfor said furnace, a conduit leading directly from the mill to saidburner for conveying pulverized fuel to the furnace by air flotation,

a feeder for introducing fuel into said mill at a controllable rate,means forforcing air under pressure into and through the mill causing aflow of air whereby to convey the pulverized fuel into the furnace byway of said conduit and said burner, variable flow resistance means tocontrol said flow of air, and control means for 7 said flow resistancemeans whichare in part responsive to the steaming load ofsaid boilerfurnace and in part responsive to the difference between the pressure inthe mill and the pressure in the furnace said control means causing saidflow resistance means to allow air to flow through said mill to saidfurnace at a rate that is adequate to permit operation of the mill tosustain said steaming load.

2. The combination of a pulverizing mill, a boiler fure.

nace subjected to variations in static pressure with a fluctuating steamload, a burner for said furnace, a conduit leading directly from themill to said burner for conpulverized fuel to the furnace by airflotation, a I

veying feeder for introducing lable rate,

fuel into said mill at a controlthe pressure in the mill and the saidcontrol means causing said air forcing means to supply air at a ratethat is adequate to permit operation of the mill to sustain saidsteaming load.

3. In a system comprising a pulverizing mill, a boiler furnace subjectedto variations in static pressure with a fluctuating steam load, a burnerfor said furnace,- a confrom the mill to said burner for 13 controls thecombinedin response to steam means for forcing air under pressure intoand through the mill whereby to convey the pulverized fuel into' thefurnace by way of said conduit and said burner, and control means forsaid air forcing means which are in part responsive to the steaming loadof said boiler furnace and in part responsive ,to the difference betweenpressure inthe furnace,

spares? means which are in part responsive to the steaming load of saidboiler furnace-and in part responsive to the diiference between thepressure in the mill and the pressure in the furnace, said feedercontrol means causing said feeder to supply fuel, and said air forcingcontrol means causing said air forcing means to supply air, both fueland air, respectively, at a rate'that is adequate to permit operation ofthe mill to-sustain said steaming load.

4. The combination of a pulverizing and fuel dryingmill, a boilerfurnace subjected to variations in static pressure with a fluctuatingsteam load, a burner for said furnace, a conduit leading directly fromthe mill to said burnerfor conveying pulverized and dried fuel to thefurnace by air flotation, a feeder for introducing moisture carryingfuel into said mill at a controllable rate, means for forcing heated airunder pressure into and through the mill whereby to dry said fuel whilebeing pulverized and to convey the dried and pulverized fuel into thefurnace by way of said conduit and said burner, and control means forsaid air forcing means which are in part responsive to the steaming loadof said boiler furnace and in part responsive to the difference betweenthe pressure in the mill and the pressure in the furnace and in partresponsive to the temperature of said conveying air as it leaves themill.

5. In a system comprising a pulverizing and fuel drying mill, a boilerfurnace subjected to a variations in static pressure with a fluctuatingsteam load, a burner for said furnace, a conduit leading directly fromthe mill to said burner for conveying pulverized and dried fuel to thefurnace by air flotation, a feeder for introducing moisture carryingfuel into said mill at a controllable rate, and means for forcing heatedair under pressure into and through the mill whereby to dry said fuelwhile being pulverized and to convey the dried and pulverized fuel intothe furnace by way of said conduit and said burner, the combination ofcontrol means for said feeder which are responsive to the steaming loadof said boiler furnace and control means for said air forcing meanswhich are in part responsive to the steaming load of said boilerfurnace, in part responsive to the difference between the pressure inthe mill and the pressure in the furnace and in part responsive to thetemperature of said conveying air as it leaves the mill.

6. The combination of a pulverizing and fuel drying mill, a boiler andfurnace subjected to variations in static pressure with a fluctuatingsteam load, a burner for said furnace, a conduit leading directly fromthe mill to said burner for conveying pulverized and dried fuel to thefurnace by air flotation, a feeder for introducing moisture carryingfuel into said mill at a controllable rate, means for forcing heated airunder pressure into and through the mill whereby to dry said fuel whilebeing pulverized and to convey the dried and pulverized fuel into thefurnace by way of said conduit and said burner, instrument means forreceiving a first control impulse responsive to the temperature of saidconveying air as it leaves the mill and for transmitting said impulse toapparatus for regulating the temperature of said heated air, instrumentmeans pulverized and to convey the dried and pulverized fuel into thefurnace by way of said'conduit and said burner,

receiving and transmitting a second control impulse responsive to thesteaming load on said boiler, instrument means for receiving andtransmitting a third control impulse responsive to the differencebetween the static pressure in the mill and the pressure in the furnace,and instrument means receiving and correlating said second and thirdcontrol impulses for regulating said air forcing means, whereby said airforcing means supply heated air at a controlled temperature and ratethat is adequate to permit operation of the mill, including suflicientdrying of said fuel to sustain said steaming load.

7. In a system comprising a pulverizing and fuel drying mill, a boilerfurnace subjected to variations in static pressure with a fluctuatingsteam load, a burner for said furnace, a conduit leading directly fromthe mill to said burner'for conveying pulverized and dried fuel to thefurnace by air flotation, a feeder for introducing moisture I carryingfuel into said mill at a controllable rate, and means for forcing heatedair under'pressure into and through the mill whereby to dry said fuelwhile being the combination of control means for said feeder which areresponsive to the steaming load of said boiler furnace, instrument meansfor receiving a first control impulse responsive to the temperature ofsaid conveying air as it leaves the mill and for transmitting saidimpulse to apparatus for regulating the temperature of said heated air,instrument means receiving and transmitting a second control impulseresponsive to the steaming load on said boiler furnace, instrument meansfor receiving and transmitting a third control impulse responsive to thedifference between the static pressure in the mill and the staticpressure in the furnace, and instrument means receiving and correlatingsaid second and third control impulses for regulating said air forcingmeans, whereby said feeder is caused to supply fuel and said air forcingmeans is caused to supply heated air at a controlled temperature, bothfuel and air, at a rate and temperature that is adequate to permitoperation of the mill, including sufficient drying of said fuel tosustain said steaming load.

8. In a fuel pulverizing and burning system for generation of steam in aboiler, the combination of a pulverizing mill including means forfeeding thereto fuel to be pulverized; means for supplying air to themill to provide a carrier for the pulverized fuel; a boiler furnaceincluding a burner for burning said fuel for the generation of steam,said furnace being subjected to variations in static pressure withfluctuations in steam load; a discharge conduit for the pulverizer andthrough which the air and pulverized fuel in suspension is passed tosaid burner; control means responsive to the steam load for varying therate of supply of fuel to the pulverizer; and automatic means forproportionally varying the rate of air supply in simultaneous responseto fuel supply and the static pressure in said furnace.

References Cited in the file of this patent Dickey Nov. 14, 1950

